Practical Methods to Estimate Horizontal and Vertical Permeabilities

ABSTRACT

In one method, the permeabilities are obtained by correcting the geometric factor derived from combining the FRA analysis and buildup analysis.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/014,422 filed on Dec. 16, 2004, which claimed priority from U.S.Provisional Patent Application Ser. No. 60/604,552 filed on 26 Aug.2004, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to the field of instruments used to samplefluids contained in the pore spaces of earth formations. Morespecifically, the invention is related to methods of determininghydraulic properties of anisotropic earth formations by interpretingfluid pressure and flow rate measurements made by such instruments.

2. Description of the Related Art

Electric wireline formation testing instruments are used to withdrawsamples of fluids contained within the pore spaces of earth formationsand to make measurements of fluid pressures within the earth formations.Calculations made from these pressure measurements and measurements ofthe withdrawal rate can be used to assist in estimating the total fluidcontent within a particular earth formation.

A typical electric wireline formation testing instrument is described,for example, in U.S. Pat. No. 5,377,755 issued to Michaels et al.Electric wireline formation testing instruments are typically loweredinto a wellbore penetrating the earth formations at one end of anarmored electrical cable. The formation testing instrument usuallycomprises a tubular probe which is extended from the instrument housingand then is impressed onto the wall of the wellbore. The probe isusually sealed on its outside diameter by an elastomeric seal or packingelement to exclude fluids from within the wellbore itself from enteringthe interior of the probe, when fluids are withdrawn from the earthformation through the probe. The probe is selectively placed inhydraulic communication, by means of various valves, with samplingchambers included in the instrument. Hydraulic lines which connect theprobe to the various sample chambers can include connection to a highlyaccurate pressure sensor to measure the fluid pressure within thehydraulic lines. Other sensors in the instrument can make measurementsrelated to the volume of fluid which has entered some of the samplechambers during a test of a particular earth formation. U.S. Pat. No.6,478,096 to Jones et al. discloses a formation pressure tester that ispart of a bottomhole assembly used in drilling and can make measurementswhile drilling (MWD).

Properties of the earth formation which can be determined usingmeasurements made by the wireline formation testing instrument includepermeability of the formation and static reservoir pressure.Permeability is determined by, among other methods, calculating a rateat which a fluid having a known viscosity moves through the pore spaceswithin the formation when a predetermined differential pressure isapplied to the formation. As previously stated, the formation testinginstrument typically includes a sensor to make measurements related tothe volume of fluid entering the sample chamber, and further includes apressure sensor which can be used to determine the fluid pressure in thehydraulic lines connecting the probe to the sample chamber. It isfurther possible to determine the viscosity of the fluid in the earthformation by laboratory analysis of a sample of the fluid which isrecovered from the sample chamber.

The permeability of a reservoir is an important quantity to know as itis one of the important factors determining the rate at whichhydrocarbons can be produced from the reservoir. Historically, two typesof measurements have been used for determination of permeability. In theso-called drawdown method, a probe on a downhole tool in a borehole isset against the formation. A measured volume of fluid is then withdrawnfrom the formation through the probe. The test continues with a buildupperiod during which the pressure is monitored. The pressure measurementsmay continue until equilibrium pressure is reached (at the reservoirpressure). Analysis of the pressure buildup using knowledge of thevolume of withdrawn fluid makes it possible to determine a permeability.

In the so-called buildup method, fluid is withdrawn from the reservoirusing a probe and the flow of fluid is terminated. The subsequentbuildup in pressure is measured and from analysis of the pressure, aformation permeability is determined.

U.S. Pat. No. 5,708,204 to Kasap having the same assignee as the presentapplication and the contents of which are fully incorporated herein byreference, teaches the Fluid Rate Analysis (FRA) method in which datafrom a combination of drawdown and buildup measurements are used todetermine a formation permeability.

The methods described above give a single value of permeability. Inreality, the permeability of earth formations is anisotropic. It is notuncommon for horizontal permeabilities to be ten or more times greaterthan the vertical permeability. Knowledge of both horizontal andvertical permeabilities is important for at least two reasons. First,the horizontal permeability is a better indicator of the productivity ofa reservoir than an average permeability determined by the methodsdiscussed above. Secondly, the vertical permeability provides usefulinformation to the production engineer of possible flow rates betweendifferent zones of a reservoir, information that is helpful in thesetting of packers and of perforating casing in a well. It is to benoted that the terms “horizontal” and “vertical” as used in the presentdocument generally refers to directions in which the permeability is amaximum and a minimum respectively. These are commonly, but notnecessarily horizontal and vertical in an earth reference frame.Similarly, the term “horizontal” in connection with a borehole is one inwhich the borehole axis is parallel to a plane defined by the horizontalpermeability.

U.S. Pat. No. 4,890,487 to Dussan et al. teaches a method fordetermining the horizontal and vertical permeabilities of a formationusing measurements made with a single probe. The analysis is based onrepresenting the fluid behavior during drawdown by an equation of theform: $\begin{matrix}{{{P_{f} - P_{i}} = \left( {\frac{Q\quad\mu}{2\pi\quad r_{p}k_{h}}{F\left( {\frac{\pi}{2},\sqrt{1 - {k_{V}/k_{H}}}} \right)}} \right)},} & (1)\end{matrix}$where

-   P_(f) represents pressure of the undisturbed formation;-   P_(i) represents pressure at the end of draw-down period i;-   Q_(i) represents volumetric flow rate during draw-down period i;-   μ represents dynamic viscosity of the formation fluid;-   r_(p) represents the probe aperture radius;-   k_(H) represents horizontal formation permeability;-   k_(V) represents vertical formation permeability; and-   F denotes the complete elliptic integral of the first kind.    In Dussan, at least three sets of measurements are made, such as two    drawdown measurements and one buildup measurement, and results from    these are combined with a table lookup to give an estimate of    vertical and horizontal permeability. The above equation was derived    based on several assumptions: an infinite wellbore, constant    drawdown rate and steady state flow. The steady state flow condition    cannot be satisfied in a low permeability formation, or unless a    long test time is used. A constant drawdown rate is not reachable in    practice because the tool needs time for acceleration and    deceleration. The storage effect also makes it difficult to reach a    constant drawdown rate. The infinite wellbore assumption excludes    the wellbore effect on the non-spherical flow pattern, making their    method not inapplicable to high k_(H)/k_(V) cases. The cases of    k_(H)/k_(V)<1 were not presented in Dussan. The method works only in    a homogeneous formation. However, their method does not have any    procedure to check if the condition of homogeneous formation can be    satisfied for a real probe test. The present invention addresses all    of these limitations.

U.S. Pat. No. 5,265,015 to Auzerais et al. teaches determination ofvertical and horizontal permeabilities using a special type of probewith an elongate cross-section, such as elliptic or rectangular.Measurements are made with two orientations of the probe, one with theaxis of elongation parallel vertical, and one with the axis ofelongation horizontal. The method requires a special tool configuration.To the best of our knowledge, there does not exist such a tool and it isprobably difficult or expensive to build one. The present invention doesnot require a special tool, and such tool is available, for example, theone described in U.S. Pat. No. 6,478,096 to Jones et al.

U.S. Pat. No. 5,703,286 to Proett et al. teaches the determination offormation permeability by matching the pressure drawdown and builduptest data (possibly over many cycles). There is a suggestion that themethod could be modified to deal with anisotropy and explicit equationsare given for the use of multiple probes. However, there is no teachingon how to determine formation anisotropy from measurements made with asingle probe. Based on the one equation given by Proett, it would beimpossible to determine two parameters with measurements from a singleprobe. It would be desirable to have a method of determination ofanisotropic permeabilities using a single probe. The present inventionsatisfies this need.

SUMMARY OF THE INVENTION

One embodiment disclosed herein is a method of estimating thepermeability of an earth formation containing a fluid. Transientpressure measurements are made with the probe in a borehole in the earthformation. The probe has a substantially circular aperture and is inhydraulic communication with the earth formation. First and secondestimates of permeability are obtained from the pressure measurements.From the first and second estimates of permeability, a horizontalpermeability and/or a vertical permeability of the formation isdetermined. The pressure measurements may include a drawdown involvingwithdrawal of the fluid from the earth formation, and stopping thewithdrawal of the fluid at a defined time. The first estimate ofpermeability may be based on analyzing a first set of pressuremeasurements made after the defined time and a second estimate ofpermeability may be based on analyzing a second set of pressuremeasurements made before the defined time. The method may furtherinvolve determining a ratio of a horizontal permeability and a verticalpermeability of the formation and/or a spherical permeability of theformation. The permeability determination may be based on using ageometric factor relating to anisotropy and/or a geometric skin factorrelating to non-spherical fluid flow. The second estimate ofpermeability may further involve matching pressure measurements madebefore and after the defined time using the first permeability and byadjusting a geometric skin factor.

Another embodiment disclosed herein is an apparatus for evaluating anearth formation which contains a fluid. The apparatus includes a probehaving a substantially circular aperture configured to be conveyed in aborehole and in hydraulic communication with the earth formation. Theapparatus includes a pressure sensing device configured to maketransient pressure measurements in the probe. The apparatus furtherincludes a processor configured to estimate a first permeability and asecond permeability from the pressure measurements, and determine fromthe first and second permeability a horizontal permeability of theformation and/or a vertical permeability of the formation. The pressuremeasurements may further include a drawdown involving the withdrawal offluid from the earth formation and stopping the withdrawal of the fluidat a defined time. A processor is configured to obtain the firstestimate of permeability at least in part by analyzing a first set ofpressure measurements made after the defined time, and obtaining thesecond estimate of permeability at least in part by analyzing pressuremeasurements made before and after the defined time. The processor mayfurther be configured to determine a ratio of horizontal permeabilityand vertical permeability and/or spherical permeability of theformation. The apparatus may further include a conveyance deviceconfigured to convey the probe into the borehole. The conveyance devicemay be a wireline or a drilling tubular.

Another embodiment disclosed herein is a computer-readable medium foruse with an apparatus for evaluating an earth formation containing aformation fluid. The apparatus includes a probe having a substantiallycircular aperture configured to be conveyed in a borehole. A pressuresensing device is configured to make transient pressure measurements inthe probe. The medium includes instructions which enable a processor toestimate a first permeability and a second permeability from thepressure measurements, and determine from the first permeability and asecond permeability a horizontal permeability of the formation and/or avertical permeability of the formation. The medium may include a ROM, anEPROM, an EEPROM, a Flash Memory, and an optical disk.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood with reference to theaccompanying figures in which like numerals refer to like elements andin which:

FIG. 1 (prior art) is an illustration of a wireline conveyed formationtesting instrument positioned within a wellbore;

FIG. 2 (prior art) shows a graph of measured pressure with respect tofluid flow rate in the earth formation;

FIG. 3 shows numerical values of the G_(os) in FRA for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V);

FIG. 4 shows numerical values of the s_(p) for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V);

FIG. 5 shows numerical values of the r_(ep) for various values ofr/r_(w) and anisotropy k_(H)/k_(V);

FIG. 6 is an FRA plot for the simulated probe test with k_(H)/k_(V)=10;

FIG. 7 is a plot of pressure changes and pressure derivatives forbuildup data;

FIG. 8 is a flow chart illustrating one embodiment of the presentinvention for determining horizontal and vertical permeabilities frombuildup and FRA analysis;

FIG. 9 is a comparison of simulated pressure data with an analyticalspherical solution derived using the buildup permeability and anisotropic skin factor;

FIGS. 10 a, 10 b shows use of a probe for two measurements in a nearhorizontal borehole;

FIG. 11 shows K values for various values of r_(p)/r_(w) and anisotropyk_(H)/k_(V);

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated schematically a section ofa borehole 10 penetrating a portion of the earth formations 11, shown invertical section. Disposed within the borehole 10 by means of a cable orwireline 12 is a sampling and measuring instrument 13. The sampling andmeasuring instrument is comprised of a hydraulic power system 14, afluid sample storage section 15 and a sampling mechanism section 16.Sampling mechanism section 16 includes selectively extensible wellengaging pad member 17, a selectively extensible fluid admittingsampling probe member 18 and bi-directional pumping member 19. Thepumping member 19 could also be located above the sampling probe member18 if desired.

In operation, sampling and measuring instrument 13 is positioned withinborehole 10 by winding or unwinding cable 12 from a hoist 19 aroundwhich cable 12 is spooled. Depth information from depth indicator 20 iscoupled to processor 21. The processor analyzes the measurements made bythe downhole tool. In one embodiment of the invention, some or all ofthe processing may be done with a downhole processor (not shown). Asatellite link 23 may be provided to send the data to a remote locationfor processing.

For any formation testing tool, the flow measurement using a singleprobe is the cheapest and quickest way. The present invention providestwo practical methods to estimate horizontal and vertical permeabilitiesfrom such probe test data. The first method is to combine the results ofthe two analyses, FRA and pressure buildup analysis. The second methodis to combine the results of buildup analysis and pressure historymatching. The probe test can be conducted using Baker Atlas's formationtesting tool used under the service mark RCI^(SM). Some details of theformation testing tool are described in U.S. Pat. No. 5,377,755 issuedto Michaels et al., having the same assignee as the present inventionand the contents of which are incorporated herein by reference.

The method of the present invention uses data from a drawdown test and apressure buildup test made with a single probe. The relationship betweenmeasured pressure and formation flow rate can be observed in the graphin FIG. 2. The pressure and flow rate measurements are shown asindividual points connected by a curve 70. A linear regression analysisof the points on curve 70 can be used to generate a line 72 for whichthe slope can be calculated. The slope of line 72 is related to thefluid mobility.

As discussed in Sheng et al., if the non-spherical flow pattern isdescribed using a geometric skin factor, s_(p), the spherical drawdownsolution may be written $\begin{matrix}{{{p_{i} - {p(t)}} = {{\frac{q\quad\mu}{4\pi\quad k_{s}r_{p}}\left( {1 + s_{p}} \right)} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{\phi\quad\mu\quad c_{t}}{\pi\quad k_{s}}}\frac{1}{\sqrt{t}}}}},} & (2)\end{matrix}$where

-   c_(t) is the total formation compressibility, atm⁻¹;-   k_(s) is the spherical permeability, D;-   p(t) represents the measured pressure in the tool, atm;-   P_(i) is the initial formation pressure, atm;-   q is the volumetric flow rate, cm³/s;-   r_(p) is the true probe radius,cm;-   s_(p) is the geometric skin factor, dimensionless;-   t is the time since the start of drawdown, s;-   μ is the viscosity of fluid, cP; and-   φ is the formation porosity, fraction.    The units of measurement are not relevant except as far as they    pertain to specific numerical values derived later in this document.

The steady-state pressure drop for a single probe in an anisotropicformation was investigated by Dussan and Sharma (1992). On the basisthat most of the pressure drop occurs in the vicinity of the probe andthe probe is very small in relation to the wellbore, they treated thewellbore as being infinite in diameter (r_(w)=∞). Their pressure drop isformulated by $\begin{matrix}{{{\Delta\quad{p\left( {\eta,r_{p},{r_{w} = \infty}} \right)}} = {\frac{q\quad\mu}{2\pi\sqrt{k_{H}k_{V}}{\max\left( {r_{p},{r_{p}/\eta}} \right)}}{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}}},} & (3)\end{matrix}$where η=k_(V)/k_(h), and F(π/2, e) is the complete elliptical integralof the first kind defined as $\begin{matrix}{{F\left( {\frac{\pi}{2},e} \right)} = {\int_{0}^{1}{\frac{\mathbb{d}v}{\sqrt{\left( {1 - v^{2}} \right)\left( {1 - {{\mathbb{e}}^{2}v^{2}}} \right)}}.}}} & (4)\end{matrix}$Note that F tends to π/2 as e defined as √{square root over (1−η²)}tends to zero in an isotropic case.

Wilkinson and Hammond (1990) extended Dussan and Sharma's work toinclude a correction for the borehole radius by introducing a shapefactor, C_(eff). The shape factor is defined as $\begin{matrix}{{C_{eff}\left( {\eta,r_{p},r_{w}} \right)} = {\frac{\Delta\quad{p\left( {\eta,r_{p},r_{w}} \right)}}{\Delta\quad{p\left( {\eta,r_{p},{r_{w} = \infty}} \right)}}.}} & (5)\end{matrix}$Then the pressure drop is $\begin{matrix}{{{\Delta\quad{p\left( {\eta,r_{p},r_{w}} \right)}} = \frac{q\quad\mu\quad{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}C_{eff}}{2\pi\sqrt{k_{H}k_{V}}{\max\left( {r_{p},{r_{w}/\eta}} \right)}}},{where}} & (6) \\{C_{eff} = {1 - {{\frac{\max\left( {r_{p},{r_{w}/\eta}} \right)}{4r_{w}{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}}\left\lbrack {3.3417 + {\ln\left( {\frac{r_{w}\eta}{2{r_{p}\left( {1 + \eta} \right)}} - \frac{1}{1 + \eta}} \right)}} \right\rbrack}.}}} & (7)\end{matrix}$When the wellbore radius tends to infinity, Ceff tends to 1, and eqn. 6becomes identical to eqn. 3, as should be the case. In the FRAformulation, non-spherical flow geometry is considered by introducing ageometric factor, G₀. The pressure drop induced by a flow rate is$\begin{matrix}{{{\Delta\quad{p\left( {\eta,r_{p},r_{w}} \right)}} = \frac{q\quad\mu}{G_{0}k_{FRA}r_{p}}},} & (8)\end{matrix}$where k_(FRA) is the permeability estimated from the FRA technique.

By comparing eqns. 6 and 8, the values of G_(o)can be derived from thevalues of F and C_(eff) using the following equation $\begin{matrix}{G_{0} = {\frac{2\pi\sqrt{k_{H}k_{V}}{\max\left( {r_{p},{r_{p}/\eta}} \right)}}{{F\left( {\frac{\pi}{2},\sqrt{1 - \eta^{2}}} \right)}C_{eff}r_{p}k_{FRA}}.}} & (9)\end{matrix}$Deriving the values of G₀ using the above equation depends on whichpermeability k_(FRA) is (horizontal, vertical or sphericalpermeability). It also involves the calculation of the completeelliptical integral. Sometimes such calculation may not be performedeasily, especially when η² is greater than one. It is also found thatthe values of C_(eff) calculated using eqn. 7 are even larger than 1.0in the cases of high k_(H)/k_(V), which violates the fluid flow physics.This is attributable to violation of one of the assumptions used in thederivation of eqn 7 when k_(H)/k_(V) is very large. In other words, eqn.7 is not applicable in some cases. As a result, we may not be able touse eqn. 6 to calculate the pressure drop in some cases.

Wilkinson and Hammond (1990) corrected the values of C_(eff) in thecases of high k_(H)/k_(V). Based on the corrected C_(eff), they definedanother parameter, k_(H)/k_(D). Here k_(H) is horizontal permeability,and k_(D) is a drawdown permeability, defined as $\begin{matrix}{k_{D} = {\frac{q\quad\mu}{4r_{p}\Delta\quad p}.}} & (10)\end{matrix}$k_(D) is computed as if the flow occurs in an isotropic formation andthe borehole is infinite. In this case, the flow is a hemi-sphericalflow, and the equivalent probe radius is 2r_(p)/π. When eqn. 10 is usedin an isotropic formation with an infinite wellbore, the estimated k_(D)is the true formation permeability. When eqn. 10 is used in a realanisotropic formation with a real finite wellbore, the estimated k_(D)may not represent the horizontal, vertical, or spherical permeabilityand is a function of k_(H)/k_(V) and r_(p)/r_(w). Because k_(D) is afunction of k_(H)/k_(V) and r_(p)/r_(w), we can use its values to derivethe values for other geometric correction factors at differentk_(H)/k_(V) and r_(p)/r_(w).

To estimate G_(o), we compare eqn. 8 with eqn. 10, and get$\begin{matrix}{{G_{0}k_{FRA}} = {{4k_{D}} = {\frac{q\quad\mu}{4r_{p}\Delta\quad p}.}}} & (11)\end{matrix}$We note for a particular test that with the measured q and αp, and thefixed μ, r_(p), the product, G_(o)k_(FRA), is fixed. G_(o) and k_(FRA)are related to each other by the relationship described by the aboveequation. In other words, depending on the type of permeability sought(e.g., horizontal, vertical, or spherical permeability), differentvalues of G_(o) are required. From eqn. 11, $\begin{matrix}{G_{0} = {\frac{4k_{D}}{k_{FRA}}.}} & (12)\end{matrix}$If a spherical permeability from FRA is sought, then it is necessary touse the G_(os) which corresponds to spherical permeability:$\begin{matrix}{G_{os} = {\frac{4}{k_{s}/k_{D}} = {\frac{4}{\left( {k_{H}^{2/3}{k_{V}^{1/3}/k_{D}}} \right)} = {\frac{4}{k_{H}/k_{D}}{\left( \frac{k_{H}}{k_{V}} \right)^{1/3}.}}}}} & (13)\end{matrix}$ $\begin{matrix}{s_{p} = {\frac{\pi\left( {k_{H}/k_{V}} \right)}{\left( {k_{H}/k_{V}} \right)^{1/3}} - 1.}} & (15)\end{matrix}$

Again from the published values of k_(H)/k_(D) (Wilkinson and Hammond,1990), the values of s_(p) are readily obtained. The values as afunction of r_(p)/r_(w) and k_(H)/k_(D) are tabulated in Table 2 andshown in FIG. 4. TABLE 2 Numerical values of s_(p) for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.0250.05 0.1 0.2 0.3 0.01 2.35 2.35 2.35 2.21 2.21 0.1 2.45 2.45 2.38 2.252.18 1 2.08 2.02 1.95 1.83 1.70 10 1.32 1.26 1.17 1.06 0.97 100 0.510.46 0.39 0.29 0.23 1000 −0.11 −0.15 −0.21 −0.26 −0.30 10000 −0.52 −0.54−0.57 −0.60 −0.62 100000 −0.75 −0.76 −0.78 −0.79 −0.80 1000000 −0.87−0.88 −0.88 −0.89 −0.90Again, there is little dependence on the probe packer size as measuredby the dimensionless probe size, r_(p)/r_(w).

If we define an equivalent probe radius, r_(ep), to account for thedeviation from true spherical flow, we can write $\begin{matrix}{{\Delta\quad p} = {\frac{q\quad\mu}{4\pi\quad k_{s}r_{sp}}.}} & (16)\end{matrix}$Comparing eqns. 10 and 16, r_(ep) can be estimated from $\begin{matrix}{r_{ep} = {\frac{{r_{p}\left( {k_{H}/k_{V}} \right)}^{1/3}}{\pi\left( {k_{H}/k_{D}} \right)}.}} & (17)\end{matrix}$

Here spherical permeability has been assumed to be given byk_(s)=√{square root over (k_(V)k_(H) ²)}. From the published values ofk_(H)/k_(D) (Wilkinson and Hammond, 1990), the values of G_(os) arereadily obtained. The values as a function of r_(p)/r_(w) andk_(H)/k_(V) are tabulated in Table 1 and shown in FIG. 3. TABLE 1Numerical values of G_(os) in FRA for various values of r_(p)/r_(w) andanisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.025 0.05 0.1 0.2 0.30.01 3.75 3.75 3.75 3.92 3.92 0.1 3.64 3.64 3.71 3.87 3.95 1 4.08 4.174.26 4.44 4.65 10 5.42 5.56 5.78 6.11 6.38 100 8.33 8.60 9.06 9.72 10.261000 14.18 14.87 15.81 17.09 18.02 10000 25.96 27.45 29.31 31.57 33.15100000 49.64 52.60 55.92 59.89 62.51 1000000 97.09 102.30 108.40 115.27119.76From Table 1 and FIG. 3, we see that the geometric factor is a strongfunction of anisotropy and a weak function of r_(p)/r_(w). Also, thevalues of G_(os) for k_(H)/k_(V) from 1 to 100 calculated from eqn. 13are in close agreement with those calculated from eqn. 9 in whichC_(eff) is calculated using eqn. 7.

The concept of geometric skin was proposed to represent the abovedefined geometric factor (Strauss, 2002). Defining a geometric skinfactor s_(p) to account for the deviation from the true spherical flowgives $\begin{matrix}{{\Delta\quad p} = {\frac{q\quad{\mu\left( {1 + s_{p}} \right)}}{4\pi\quad k_{s}r_{p}}.}} & (14)\end{matrix}$Comparing eqn. 10 and eqn. 14, s_(p) can be estimated from

Using the published values of k_(H)/k_(D) (Wilkinson and Hammond, 1990),the values of r_(ep) are readily obtained. The values as a function ofr_(p)/r_(w) and k_(H)/k_(V) are tabulated in Table 3 and shown in FIG.5. TABLE 3 Numerical values of the r_(ep)/r_(p) for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.0250.05 0.1 0.2 0.3 0.01 0.30 0.30 0.30 0.31 0.31 0.1 0.29 0.29 0.30 0.310.31 1 0.32 0.33 0.34 0.35 0.37 10 0.43 0.44 0.46 0.49 0.51 100 0.660.68 0.72 0.77 0.82 1000 1.13 1.18 1.26 1.36 1.43 10000 2.07 2.18 2.332.51 2.64 100000 3.95 4.19 4.45 4.77 4.97 1000000 7.73 8.14 8.63 9.179.53The formulation and values of the above correction factors are based onthe related spherical flow eqns. 8, 14, or 16. For eqn. 6, k_(FRA) isassumed to be the spherical permeability. Comparing their defining eqns.11, 13, and 15, it can be seen that these three correction factors havethe following relationship: $\begin{matrix}{{{{s_{p} + 1} = {\frac{4\pi}{G_{os}} = \frac{1}{r_{ep}/r_{p}}}};}{or}} & (18) \\{{{s_{p} + 1} = \frac{4\pi}{G_{os}}},} & (19) \\{\frac{1}{r_{ep}/r_{p}} = {\frac{4\pi}{G_{os}}.}} & \left( {19a} \right)\end{matrix}$

Substituting from eqn. 19 into eqn. 2, gives: $\begin{matrix}{{p_{i} - {p(t)}} = {\frac{q\quad\mu}{G_{os}k_{s}r_{p}} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{\phi\quad\mu\quad c_{t}}{\pi\quad k_{s}}}{\frac{1}{\sqrt{t}}.}}}} & (20)\end{matrix}$Eqns. 2 and 20 are valid for both isotropic and anisotropic formations.Using the principle of superposition, the buildup solution is$\begin{matrix}{{{p(t)} = {p_{i} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{\phi\quad\mu\quad c_{t}}{\pi\quad k_{s}}}\left( {\frac{1}{\sqrt{\Delta\quad t}} - \frac{1}{\sqrt{t}}} \right)}}},} & (21)\end{matrix}$where αt is the shut-in time, s. Here, q is the flow rate for theprevious drawdown measurement. According to eqn. 20, the buildupmobility is estimated from $\begin{matrix}{{\left( \frac{k_{s}}{\mu} \right)_{BU} = {\frac{1}{\pi}\left( \frac{q}{4m_{s}} \right)^{2/3}\left( {\phi\quad c_{t}} \right)^{1/3}}},} & (22)\end{matrix}$where m_(s), is the slope of the linear plot of p(t) vs. the timefunction (αt^(−1/2)−t^(−1/2)).For the purposes of the present invention, the permeability measuredusing the buildup measurements is referred to as a first permeability.

Turning now to the FRA method as described in Kasap, $\begin{matrix}{{{p(t)} = {p_{i} - \frac{q_{f}\mu}{k_{s}G_{os}r_{p}}}},} & (23)\end{matrix}$where q_(f), the formation flow rate at the sand face near the probe, is$\begin{matrix}{{q_{f} = {{c_{sys}V_{sys}\frac{\mathbb{d}{p(t)}}{\mathbb{d}t}} + q_{dd}}},} & (24)\end{matrix}$corrected for the storage effect. In the above equation, C_(sys) is thecompressibility of the fluid in the tool, atm⁻¹; q_(dd) is the pistonwithdrawal rate, cm³/s ; V_(sys) is the system (flow line) volume, cm³.

According to FRA, the data in both drawdown and buildup periods arecombined to estimate the mobility from $\begin{matrix}{{\left( \frac{k_{s}}{\mu} \right)_{FRA} = \frac{1}{G_{os}r_{p}m_{FRA}}},} & (25)\end{matrix}$where m_(FRA) is the slope of the linear plot of p(t) vs. q_(f). Byplotting the drawdown data and the buildup data in the FRA plot (FIG.2), if both data are seen to fall on the same straight line with aslope, m_(FRA), the estimated permeability from the drawdown and thebuildup is the same. That means within the radius of investigation forthe drawdown and buildup, the formation is homogeneous. This is thecondition for the presented methods to work.

Eqn. 25 shows that the estimated mobility from FRA is affected by thelocal flow geometry indicated by G_(os). Thus, a correct value of G_(os)must be provided. However, G_(os) strongly depends on the ratio ofvertical-to-horizontal permeability that is generally unknown before thetest is performed. In this case, the value of G_(os) in an isotropicformation is used. As a result, the FRA estimated permeability may notrepresent the true spherical permeability. In contrast, the sphericalpermeability can be obtained from a buildup analysis without priorknowledge of formation anisotropy, and the estimate of mobility from thebuildup analysis is not affected by the local flow geometry according toeqn. 22. In other words, the correct estimate of spherical permeabilitycan be obtained from buildup analysis without knowing formationanisotropy and local flow geometry. The difference in the estimatedspherical permeability from buildup analysis and FRA, discussed in theabove, can be used to estimate the horizontal and verticalpermeabilities. For the purposes of the present invention, thepermeability determined by FRA processing is referred to as a secondpermeability.

The difference in the estimated spherical permeability from buildupanalysis (the first permeability) and FRA permeability (the secondpermeability), discussed in the above, can be used to estimate thehorizontal and vertical permeabilities. A simulated probe-pressure testdata as an example is used to illustrate the procedures. First, theprobe-pressure test simulation is described.

The simulation model used is given in Table 4. TABLE 4 Input parametersused in simulation Porosity, fraction 0.2 Spherical permeability, mD 10k_(H)/k_(V) 10 Viscosity, cP 1 Formation pressure, psi 4000 Fluidcompressibility, 1/psi 2.50E−06 Wellbore radius, cm. 6.35 Probe radius,cm 0.635 Flow line volume, ml 371 Drawdown rate, ml/s 4 Duration ofdrawdown, s 10The symmetry in the problem is used to reduce the model to one quarterof the probe and the formation. Further, the effect of gravity isneglected. The model is a radial model. The k_(H)/k_(V) is equal to 10with the spherical permeability of 10 mD. The r_(p)/r_(w) is equal to0.1. The drawdown rate for the quarter model is 1 ml/s.

Next, results of analyzing the simulation data using the FRA techniqueare discussed. FIG. 6 shows the expected linear relation between thepressure and the formation flow rate. If the data were realprobe-pressure test data and k_(H)/k_(V) were unknown, one couldlogically assume the formation were isotropic. According to Table 1, thegeometric factor, G_(os), would be 4.26 for r_(p)/r_(w) equal to 0.1.Based on eqn. 25 and using this value of G_(os), one would estimate aspherical permeability of 13 mD.

The simulated pressure test data could also be analyzed using buildup(BU) analysis using any pressure transient analysis software withspherical flow solutions. For this example, the commercially availablesoftware Interpret2003 of Paradigm Geophysical Co was used. FIG. 7 showsthe buildup analysis plot to estimate the spherical permeability forthis case. The abscissa is time and ordinate is the pressure change 701or the pressure derivative 703. The plot is on a log-log scale. Alsoshown on the plot are lines with a slop of +1 (705) and a slope of −½(707). The spherical flow regime is identified by a negative half slopein the log-log derivative plot. From this buildup analysis, thespherical permeability is estimated to be 9.62 mD, close to the inputspherical permeability. It should be noted that the use of theInterpret2003 software is for exemplary purposes only and other softwarepackages that perform similar functions (as described below) could beused.

For the same pressure data, different estimates of permeability areobtained from buildup analysis and from FRA. One is 13 mD from FRA, theother is 9.62 mD from the BU analysis. The latter is close to the actualpermeability used in the simulation model. The former is different fromthe actual permeability because we used an incorrect G_(os). To make FRAestimated permeability closer to the actual one used in the simulation,a value of G_(os) appropriate for the permeability anisotropy ratio inthe simulation should be used. Assuming the BU estimated sphericalpermeability is correct, the correct G_(os) can be estimated as follows.$\begin{matrix}{{\left( {k_{s}G_{os}} \right)_{FRA} = \frac{\mu}{r_{p}m_{FRA}}},} & (26)\end{matrix}$The above shows that for a particular test, since the linearrelationship between the measured q and αp results in a constant slope,m_(FRA), for the fixed μ and r_(p), the product, (G_(os)k_(s))FRA, isfixed. In other words, for a particular test, if an isotropic formationis assumed for FRA, then (G_(os)k_(s)) in the isotropic formation,denoted by (G_(os)k_(s))_(iso), should equal the permeability-geometricfactor product of the anisotropic formation, (G_(os)k_(s))_(ani). Thisproduct consists of the correct G_(os) and the correct k_(s) in theanisotropic formation. Because the BU estimated permeability is assumedto be the true spherical permeability, then the correct G_(os) in theanisotropic formation, (G_(o))_(ani), can be estimated from$\begin{matrix}{\left( G_{os} \right)_{ani} = {\frac{\left( {G_{os}k_{s}} \right)_{iso}}{\left( k_{s} \right)_{BU}}.}} & (27)\end{matrix}$

In the term (G_(os)k_(s))_(iso) of the above equation, G_(os) is thegeometric factor for an isotropic formation (G_(os)=4.26 from Table 1),and k_(s) is the FRA permeability estimated initially assuming theformation is isotropic. For this example, k_(s) is 13 mD. In thedenominator, (k_(s))_(BU) is the spherical permeability estimated fromthe buildup analysis which is 9.62 mD in this example. Therefore, thecorrect G_(os) in this example is $\begin{matrix}{\left( G_{os} \right)_{ani} = {\frac{\left( {G_{os}k_{s}} \right)}{\left( k_{s} \right)_{BU}} = {\frac{(4.26)(13)}{9.62} = {5.76.}}}} & (28)\end{matrix}$The estimated G_(os) of 5.76 is very close to the G_(os) in FIG. 1 whenk_(H)/k_(V) is equal to 10 and r_(p)/r_(w) is equal to 0.1. Therefore,by combining the results of FRA and buildup analysis, it is possible todetermine k_(H)/k_(V). Having k_(H)/k_(V) determined, the horizontalpermeability and vertical permeability are readily obtained:k_(H)=(k_(s))_(BU)/(k_(H)/k_(V))^((1/3)),  (29),k_(V)k_(H)/(k_(H)/k_(V)).  (30).

For this example, the calculated horizontal permeability and verticalpermeability are 20.7 mD and 2.07 mD, respectively. These values arevery close to their respective simulation model input values of 21.54 mDand 2.15 mD. Thus the method to combine FRA and buildup analysis isdemonstrated.

FIG. 8 is a flow chart illustrating the first embodiment of theinvention. Pressure buildup data 751 are analyzed to get a firstestimate of spherical permeability. Separately, the pressure buildupdata 751 and the drawdown data 757 are analyzed to get a second estimateof spherical permeability 759. Using the two different permeabilities,the geometric factor G_(os) for the probe is corrected 755 and using thecorrected G_(os), the horizontal and vertical permeabilities aredetermined as discussed above.

A second embodiment of the present invention uses the sphericalpermeability obtained from the pressure buildup test (the firstpermeability) as a starting point for matching the entire pressurehistory, including the drawdown data. In Interpret2003 the geometricskin factor, s_(p), is used to describe the non-spherical flow near theprobe. Even though the local geometry near the probe does not affect thepermeability estimate, it does affect the pressure data as given by eqn.2. In the above example, using the BU estimated permeability of 9.62 mDand an isotropic geometric skin factor of 1.95 shown in Table 1, thepressure data from Interpret2003 cannot be matched with the simulatedpressure data because we used the wrong isotropic geometric skin factor.This is shown in FIG. 9 where the abscissa is time and the ordinate ispressure. The buildup portion is used to derive the permeability andthis derived permeability is used to model the pressure data. Moreobviously, the modeled drawdown data 803 does not match the actualdrawdown data 801. To match the simulated pressure data, it is necessaryto use the BU estimated spherical permeability, and also to change thevalue of s_(p) until the pressure data from Interpret2003 matches thenumerical simulation data. It is found that using a value of s_(p) equalto 1.2, a good match is obtained (not shown). From Table 2 it can beseen that s_(p) equal to 1.2 (close the s_(p) of 1.17 in Table 1)corresponds to k_(H)/k_(V) equal to 10 and r_(p)/r_(w) equal to 0.1. Asabove, k_(H)/k_(V) has been estimated to be equal to 10. Oncek_(H)/k_(V) is obtained, eqns 28 and 29 can be used to estimate thehorizontal and vertical permeabilities. Thus, the second method alsouses a permeability from BU analysis (the first method) in combinationwith matching the entire pressure data (processing of data over theentire time interval including drawdown and buildup) to estimatehorizontal and vertical permeabilities.

Conceptually, the second method is based on deriving a sphericalpermeability based on a buildup analysis, and then using this determinedspherical permeability to match the pressure history data by adjustingthe geometric skin factor. Knowledge of the spherical permeability andthe geometric skin factor makes it possible to determine the horizontaland vertical permeabilities.

The two embodiments of the present invention discussed above are used toestimate horizontal and vertical permeabilities based on the assumptionof a homogeneous and anisotropic formation. Such an assumption isreasonable in a practical probe test, because the formation on the smallscale near the probe probably can be considered virtually homogeneous.Therefore, the invention provides a way to estimate the horizontal andvertical permeabilities from a single probe test without additionalinformation. This is in contrast to prior art methods that requiresimultaneous measurements with multiple probes, or measurements with aspecially designed probe in two orientations.

In another embodiment of the invention, two tests are made in a nearhorizontal borehole. In one test, the probe is set and sealedhorizontally against a side wall of the borehole. This is schematicallyillustrated in FIG. 10 a wherein the borehole 851 is shown incross-section and a probe 853 is in contact with the side wall of theborehole. In a second test, schematically illustrated in FIG. 10 b, theprobe 853 is shown against the upper wall of the borehole. It is to benoted that the method is equally applicable if, in the second test, theprobe is against the bottom wall of the borehole.

The solution for the first test is the same as that in a vertical well,and has been discussed above. The solution for the second test isderived next. The objective is to determine the relationship between thepressure at the probe and the fluid withdrawal rate from the anisotropicformation. As before, a cylindrical coordinate system is used in whichthe wellbore wall near the probe can be approximated by the z=0 plane,with the formation located in the half-space z≧0. The initial formationpressure is p_(i). The z axis for the test of FIG. 10 b coincides withthe vertical direction. The perimeter of the probe opening through whichfluid flows is given by r²=r_(p) ² at z=0. The flowing pressure at theprobe opening is p_(p). There is no flow across the rest of the plane atz=0. The mathematical description of such probe test is a mixed boundaryproblem. Its formulation is given as follows. $\begin{matrix}{{{{k_{H}\left( {\frac{\partial^{2}p}{\partial r^{2}} + {\frac{1}{r}\frac{\partial p}{\partial r}}} \right)} + {k_{V}\frac{\partial^{2}p}{\partial z^{2}}}} = 0},} & (31) \\{{p = {{{p_{p}\quad{at}\quad r} \leq {r_{p}\quad{and}\quad z}} = 0}},} & (32) \\{{\frac{\partial p}{\partial z} = {{{0\quad{at}\quad r} > {r_{p}\quad{and}\quad z}} = 0}},} & (33) \\{\left. p\rightarrow\left. {{p_{i}\quad{as}\quad r^{2}} + z^{2}}\rightarrow{{\infty\quad{and}\quad z} \geq 0} \right. \right.,} & (34)\end{matrix}$

Of interest is the relationship between pressure drop, p_(i)-p_(p), andflow rate, q. This is done by evaluating the integral: $\begin{matrix}{q = \left. {\frac{2\pi\quad k_{V}}{\mu}{\int_{A_{p}}^{\quad}\frac{\partial p}{\partial z}}} \middle| {}_{z = 0}{r{{\mathbb{d}r}.}} \right.} & (35)\end{matrix}$In the above equations,

-   A_(p) represents area of probe opening, cm²-   k_(H) represents horizontal permeability, D-   k_(V) represents vertical permeability, D-   p represents pressure, atm-   p_(i) represents initial formation pressure, atm-   p_(p) represents pressure at the probe, atm-   q represents volumetric flow rate, cm³/s-   r represents radial coordinate of cylindrical grid system, cm-   r_(p) represents true probe radius, cm-   z represents z axis in the coordinate system, cm-   μ represents viscosity of fluid, cP    The units of measurement are not relevant except as far as they are    consistently follow one unit system. Here Darcy unit system is used.

Using the following notation: $\begin{matrix}{{r^{\prime} = r},} & (36) \\{{z^{\prime} = {\sqrt{\frac{k_{H}}{k_{V}}}z}},} & (37)\end{matrix}$the above mathematical formulation (Eqns. 31 to 35) is converted in thefollowing formulation: $\begin{matrix}{{{\left( {\frac{\partial^{2}p}{\partial r^{\prime\quad 2}} + {\frac{1}{r^{\prime}}\frac{\partial p}{\partial r^{\prime}}}} \right) + \frac{\partial^{2}p}{\partial z^{\prime\quad 2}}} = 0},} & \left( 31^{\prime} \right) \\{{p = {{{p_{p}\quad{at}\quad r^{\prime}} \leq {r_{p}\quad{and}\quad z^{\prime}}} = 0}},} & \left( 32^{\prime} \right) \\{{\frac{\partial p}{\partial z^{\prime}} = {{{0\quad{at}\quad r^{\prime}} > {r_{p}\quad{and}\quad z^{\prime}}} = 0}},} & \left( 33^{\prime} \right) \\{\left. p\rightarrow\left. {{p_{i}\quad{as}\quad r^{\prime 2}} + \frac{z^{\prime 2}}{k_{H}/k_{V}}}\rightarrow{{\infty\quad{and}\quad z^{\prime}} \geq 0} \right. \right.,} & \left( 34^{\prime} \right) \\{q = \left. {\frac{2\pi\quad\sqrt{k_{H}k_{V}}}{\mu}{\int_{A_{p}}^{\quad}\frac{\partial p}{\partial z^{\prime}}}} \middle| {}_{z = 0}{r^{\prime}{{\mathbb{d}r^{\prime}}.}} \right.} & \left( 35^{\prime} \right)\end{matrix}$

The solution for the above problem was solved by Carslaw, H. S. andJaeger, J. C., Conduction of Heat in Solids, Oxford University Press(1959). According to their solution, the relationship between pressuredrop and flow rate for the above problem is $\begin{matrix}{q = {\frac{4\sqrt{k_{H}k_{V}}{r_{p}\left( {p_{i} - p_{p}} \right)}}{\mu}.}} & (38)\end{matrix}$Note that from the above equation, it is possible to obtain apermeability (k_(H)k_(V))^(1/2), a geometric average permeability ofhorizontal permeability and vertical permeability.

For the first test with the probe set horizontally against the side wall(FIG. 10 a) in a horizontal well, the relationship between the pressuredrop and flow rate is the same as that in a vertical well. Using thegeometric factor and horizontal permeability, the relationship derivedabove is $\begin{matrix}{{{p_{i} - p_{p}} = \frac{q\quad\mu}{G_{oH}k_{H}r_{p}}},} & (39)\end{matrix}$

where G_(oH) is the geometric factor when the pressure drop vs. flowrate relationship is formulated using horizontal permeability, k_(H).Its values at different k_(H)/k_(V) and r_(p)/r_(w) are reported in thesame reference and reprinted here in Table 5. Here r_(w) is the radiusof wellbore. Note that the values in Table 5 are for G_(0H), related toa horizontal permeability whereas the values in Table 1 are for G_(0S),related to a spherical permeability. TABLE 5 Numerical values of G_(oH)(for k_(H)) for various values of r_(p)/r_(w) and anisotropy k_(H)/k_(V)r_(p)/r_(w) = k_(H)/k_(V) 0.025 0.05 0.1 0.2 0.3 0.01 17.39 17.39 17.3918.18 18.18 0.1 7.84 7.84 8.00 8.33 8.51 1 4.08 4.17 4.26 4.44 4.65 102.52 2.58 2.68 2.84 2.96 100 1.79 1.85 1.95 2.09 2.21 1000 1.42 1.491.58 1.71 1.80 10000 1.20 1.27 1.36 1.47 1.54 100000 1.07 1.13 1.20 1.291.35 1000000 0.97 1.02 1.08 1.15 1.20

From Eqn. 39, the horizontal permeability can be obtained. But thispermeability is closely related to the geometric factor which is astrong function of k_(H)/k_(V). Before analyzing the test data,k_(H)/k_(V) is unknown. However, for a particular test with the measuredq and p_(p), and the fixed μ, r_(p), the product G_(oH)k_(H) is adetermined quantity. For the second test in a horizontal well when theprobe is set vertically against the top wall of the borehole (FIG. 10b), the relationship between the pressure drop and flow rate is describeby Eqn. 38 and a mean permeability, (k_(H)k_(V))^(1/2) can be obtained.In other words, when the two tests are conducted at the same measureddepth, the following two quantities are obtained: $\begin{matrix}{{{K_{S} \equiv {G_{oH}k_{H}}} = \frac{q_{S}\mu}{r_{p}\left( {p_{i} - p_{p,S}} \right)}},} & (40) \\{{{K_{T} \equiv \sqrt{k_{H}k_{V}}} = \frac{q_{T}\mu}{4{r_{p}\left( {p_{i} - p_{p,T}} \right)}}},} & (41)\end{matrix}$where the subscripts S and T means the probe is set horizontally againstthe side wall and vertically against the top wall, respectively. BothK_(S) and K_(T) are functions of permeability anisotropy, k_(H)/k_(V).Now we define another quantity K using these two quantities:$\begin{matrix}{{K \equiv \frac{K_{S}}{K_{T}}} = {G_{o\quad H}{\sqrt{\frac{k_{H}}{k_{V}}}.}}} & (12)\end{matrix}$

Because G_(oH) is a function of k_(H)/k_(V), K is also a function ofk_(H)/k_(V). Using the values of G_(oH) in Table 4, the values of K areobtained as shown in Table 6 and FIG. 11 as a function of r_(p)/r_(w)and k_(H)/k_(V). For the two pretests conducted at the same measureddepth, the K value can be calculated using K_(S) and K_(T) from Eqns. 10and 11. Then the k_(H)/k_(V) at the measured depth can be obtained bylooking up Table 6 or FIG. 11 using the calculated K value and the knownvalue of r_(p)/r_(w). From knowledge of k_(H)/k_(V), the horizontal andvertical permeabilities are readily determined: $\begin{matrix}{{k_{H} = {K_{T}\sqrt{\frac{k_{H}}{k_{V}}}}},} & (43) \\{k_{V} = {\frac{k_{H}}{\left( {k_{H}/k_{V}} \right)}.}} & (44)\end{matrix}$ TABLE 6 Numerical values of K for various values ofr_(p)/r_(w) and anisotropy k_(H)/k_(V) r_(p)/r_(w) = k_(H)/k_(V) 0.0250.05 0.1 0.2 0.3 0.01 1.74 1.74 1.74 1.82 1.82 0.1 2.48 2.48 2.53 2.642.69 1 4.08 4.17 4.26 4.44 4.65 10 7.96 8.16 8.49 8.97 9.37 100 17.9418.52 19.51 20.94 22.10 1000 44.86 47.02 50.00 54.06 56.98 10000 120.48127.39 136.05 146.52 153.85 100000 338.21 358.33 381.00 408.04 425.901000000 970.87 1023.02 1084.01 1152.74 1197.60

The above equations are derived based on the assumptions of a constantwithdrawal rate and steady state flow. In a low permeability formation,the steady state flow condition cannot be satisfied unless a long testtime is used. A constant drawdown rate is not reachable in practicebecause the tool needs time for acceleration and deceleration. Thestorage effect also makes it difficult to reach a constant rate. In analternate embodiment of the present invention, both drawdown and builduptests are made at substantially the same depth with the probe against asidewall and an upper (or lower) wall. The Formation Rate Analysis (FRA)presented in U.S. Pat. No. 5,708,204 to Kasap, the contents of which areincorporated herein by reference, are used to calculate the above K_(S)and K_(T).

As disclosed in Kasap, pressure is measured with respect to time at aprobe in hydraulic communication with the earth formation. Timederivatives of the measured pressure are determined while withdrawingfluid from the earth formation by increasing a volume of a chamber inhydraulic communication with the probe. The volume of the chamber ismeasured as a function of time and the time derivative of the volume iscalculated. This is part of a withdrawal phase. Fluid withdrawal isstopped and the pressure buildup is measured. The permeability iscalculated when the measured pressure substantially stops increasing, bydetermining a slope of a linear relationship of the measured pressurewith respect to a fluid flow rate calculated from the time derivative ofthe volume and the time derivative of the pressure.

The invention has been described in terms of measurements made usinglogging tools conveyed on a wireline in a borehole. As noted above, Themethod can also be used on data obtained usingmeasurement-while-drilling sensors on a bottomhole assembly (BHA)conveyed by a drilling tubular. Such a device is described, for example,in U.S. Pat. No. 6,640,908 to Jones et al., and in U.S. Pat. No.6,672,386 to Krueger et al., having the same assignee as the presentinvention and the contents of which are fully incorporated herein byreference. The method disclosed in Krueger comprises conveying a toolinto a borehole, where the borehole traverses a subterranean formationcontaining formation fluid under pressure. A probe is extended from thetool to the formation establishing hydraulic communication between theformation and a volume of a chamber in the tool. Fluid is withdrawn fromthe formation by increasing the volume of the chamber in the tool with avolume control device. Data sets are measured of the pressure of thefluid and the volume of the chamber as a function of time.

The embodiments of the invention that require making measurements on twodifferent walls of a substantially horizontal borehole are readilyaccomplished in a MWD implementation. If the tests are performed afterthe well has been drilled, several options are available. One is toconvey the pressure tester on coiled tubing. Alternatively, a downholetraction device such as that disclosed in U.S. Pat. No. 6,062,315 toReinhardt, having the same assignee as the present invention and thecontents of which are fully incorporated herein by reference, may beused to convey the pressure tester into the borehole. A traction devicemay also be used to withdraw the pressure tester from the borehole, or,alternatively, the withdrawal may be done using a wireline.

The processing of the measurements made by the probe in wirelineapplications may be done by the surface processor 21 or may be done by adownhole processor (not shown). For MWD applications, the processing maybe done by a downhole processor that is part of the BHA. This downholeprocessing reduces the amount of data that has to be telemetered.Alternatively, some or part of the data may be telemetered to thesurface. In yet another alternative, the pressure and flow measurementsmay be stored on a suitable memory device downhole and processed whenthe drillstring is tripped out of the borehole.

The operation of the probe may be controlled by the downhole processorand/or the surface processor. Implicit in the control and processing ofthe data is the use of a computer program implemented on a suitablemachine readable medium that enables the processor to perform thecontrol and processing. The machine readable medium may include ROMs,EPROMs, EAROMs, Flash Memories and Optical disks.

While the foregoing disclosure is directed to the specific embodimentsof the invention, various modifications will be apparent to thoseskilled in the art. It is intended that all such variations within thescope and spirit of the appended claims be embraced by the foregoingdisclosure.

1. A method of estimating a permeability of an earth formation, theformation containing a formation fluid, the method comprising: (a)making transient pressure measurements with a probe in a borehole in theearth formation, the probe having a substantially circular aperture andbeing in hydraulic communication with the earth formation; (b) obtaininga first and a second estimate of permeability from the pressuremeasurements; and (c) determining from the first and second estimates ofpermeability at least one of (A) a horizontal permeability of theformation, and (B) a vertical permeability of the formation.
 2. Themethod of claim 1 wherein the pressure measurements further comprise:(i) a drawdown involving withdrawal of fluid from said earth formationby increasing a volume of a chamber in hydraulic communication with saidprobe and measuring said volume; and (ii) stopping the withdrawal of thefluid at a defined time; wherein obtaining the first estimate ofpermeability further comprises analyzing a first set of pressuremeasurements made after the defined time and wherein obtaining thesecond estimate of permeability further comprises analyzing measurementsmade before and after the defined time
 3. The method of claim 1 furthercomprising determining at least one of (I) a ratio of a horizontalpermeability and a vertical permeability of the formation, and (II) aspherical permeability of the formation.
 4. The method of claim 1wherein step (c) further comprises using at least one of (i) a geometricfactor relating to anisotropy, and (ii) a geometric skin factor relatingto non-spherical fluid flow.
 5. The method of claim 2 wherein obtainingthe second estimate of permeability further comprises matching pressuremeasurements made before and after said defined time using the firstpermeability and by adjusting a geometric skin factor.
 6. The method ofclaim 1 wherein determining said first permeability further comprisesusing a relation of the form:${p(t)} = {p_{i} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{{\phi\mu}\quad c_{t}}{\pi\quad k_{s}}}\left( {\frac{1}{\sqrt{\Delta\quad t}} - \frac{1}{\sqrt{t}}} \right)}}$C_(t) is a total formation compressibility; k_(s) is a sphericalpermeability; p(t) represents a measured pressure in the tool; P_(i) isthe initial formation pressure; q is a volumetric flow rate; r_(p) is atrue probe radius; t is a time after said defined time; μ is a viscosityof fluid; and φ is a formation porosity, fraction.
 7. The method ofclaim 1 further comprising conveying the probe into the borehole on oneof (i) a wireline, (ii) a drilling tubular, and (iii) a slickline. 8.The method of claim 1 wherein the borehole is substantially vertical andthe probe is oriented with its axis in any direction.
 9. The method ofclaim 1 wherein the borehole comprises a deviated borehole.
 10. Themethod of claim 9 wherein the probe aperture lies substantially in avertical plane.
 11. An apparatus for evaluating an earth formation, theformation containing a formation fluid, the apparatus comprising: (a) aprobe having a substantially circular aperture configured to be conveyedin a borehole and in hydraulic communication with the earth formation(b) a pressure sensing device configured to make transient pressuremeasurements in said probe; and (c) a processor which is configured to:(A) estimate a first permeability and a second permeability from thepressure measurements, and (B) determine from the first and secondpermeability at least one of (I) a horizontal permeability of theformation, and (II) a vertical permeability of the formation.
 12. Theapparatus of claim 11 wherein the pressure measurements furthercomprise: (i) a drawdown involving withdrawal of fluid from the earthformation by increasing a volume of a chamber in hydraulic communicationwith the probe and measuring said volume; and (ii) stopping thewithdrawal of the fluid at a defined time; wherein the processor isconfigured to obtain the first estimate of permeability at least in partby analyzing a first set of pressure measurements made after the definedtime and wherein the processor is configured to obtain the secondestimate of permeability at least in part by analyzing measurements madebefore and after the defined time
 13. The apparatus of claim 11 whereinthe processor is further configured to determine at least one of (I) aratio of a horizontal permeability and a vertical permeability of theformation, and (II) a spherical permeability of the formation.
 14. Theapparatus of claim 11 wherein the processor is configured to performstep (B) by using at least one of (i) a geometric factor relating toanisotropy, and (ii) a geometric skin factor relating to non-sphericalfluid flow.
 15. The apparatus of claim 12 wherein the processor isconfigured to obtain the second estimate of permeability at least inpart by matching pressure measurements made before and after saiddefined time using the first permeability and by adjusting a geometricskin factor.
 16. The apparatus of claim 11 wherein said processor isconfigured to determine the first permeability further using a relationof the form below:${p(t)} = {p_{i} - {\frac{q\quad\mu}{4\pi\quad k_{s}}\sqrt{\frac{\phi\quad\mu\quad c_{t}}{\pi\quad k_{s}}}\left( {\frac{1}{\sqrt{\Delta\quad t}} - \frac{1}{\sqrt{t}}} \right)}}$C_(t) is a total formation compressibility; k_(s) is a sphericalpermeability; p(t) represents a measured pressure in the tool; P_(i) isthe initial formation pressure; q is a volumetric flow rate; r_(p) is atrue probe radius; t is a time after said defined time; μ is a viscosityof fluid; and φ is a formation porosity, fraction.
 17. The apparatus ofclaim 11 further comprising a conveyance device configured to convey theprobe into the borehole, the conveyance device selected from (i) awireline, and (ii) a drilling tubular.
 18. The apparatus of claim 11wherein the borehole is substantially vertical and the probe is orientedwith its axis in any direction.
 19. The apparatus of claim 11 whereinthe borehole comprises a deviated borehole.
 20. The apparatus of claim19 wherein the probe aperture lies substantially in a vertical plane.21. A computer readable medium for use with an apparatus for evaluatingan earth formation, the formation containing a formation fluid, theapparatus comprising: (a) a probe having a substantially circularaperture configured to be conveyed in a borehole, the probe being inhydraulic communication with the earth formation; and (b) a pressuresensing device configured to make transient pressure measurements insaid probe; the medium comprising instructions which enable a processorto (c) estimate a first permeability and a second permeability from thepressure measurements, and (d) determine from the first and secondpermeability at least one of (I) a horizontal permeability of theformation, and (II) a vertical permeability of the formation.
 22. Themedium of claim 21 further comprising at least one of: (i) a ROM, (ii)an EPROM, (iii) an EEPROM, (iv) a Flash Memory, and (v) an optical disk.